The present invention relates generally to a method of making non-linear optical polymer films. Additional layers of polymer or metal may be added under vacuum as well.
As used herein, the term xe2x80x9c(meth)acrylicxe2x80x9d is defined as xe2x80x9cacrylic or methacrylic.xe2x80x9d Also, xe2x80x9c(meth)acylatexe2x80x9d is defined as xe2x80x9cacrylate or methacrylate.xe2x80x9d
As used herein, the term xe2x80x9ccryocondensexe2x80x9d and forms thereof refer to the physical phenomenon of a phase change from a gas phase to a liquid phase upon the gas contacting a surface having a temperature lower than a dew point of the gas.
As used herein, the term xe2x80x9cconjugatedxe2x80x9d refers to a chemical structure of alternating single and double bonds between carbon atoms in a carbon atom chain.
As used herein, the term xe2x80x9cpolymer precursorxe2x80x9d includes monomers, oligomers, and resins, and combinations thereof. As used herein, the term xe2x80x9cmonomerxe2x80x9d is defined as a molecule of simple structure and low molecular weight that is capable of combining with a number of like or unlike molecules to form a polymer. Examples include, but are not limited to, simple acrylate molecules, for example, hexanedioldiacrylate, and tetraethyleneglycoldiacrylate, styrene, methyl styrene, and combinations thereof. The molecular weight of monomers is generally less than 1000, while for fluorinated monomers, it is generally less than 2000. Substructures such as CH3, t-butyl, and CN can also be included. Monomers may be combined to form oligomers and resins, but do not combine to form other monomers.
As used herein, the term xe2x80x9coligomerxe2x80x9d is defined as a compound molecule of at least two monomers that may be cured by radiation, such as ultraviolet, electron beam, or x-ray, glow discharge ionization, and spontaneous thermally induced curing. Oligomers include low molecular weight resins. Low molecular weight is defined herein as about 1000 to about 20,000 exclusive of fluorinated monomers. Oligomers are usually liquid or easily liquifiable. Oligomers do not combine to form monomers.
As used herein, the term xe2x80x9cresinxe2x80x9d is defined as a compound having a higher molecular weight (generally greater than 20,000) which is generally solid with no definite melting point. Examples include, but are not limited to, polystyrene resin, epoxy polyamine resin, phenolic resin, and acrylic resin (for example, polymethylmethacrylate), and combinations thereof.
The basic process of plasma enhanced chemical vapor deposition (PECVD) is described in THIN FILM PROCESSES, J. L. Vossen, W. Kern, editors, Academic Press, 1978, Part IV, Chapter IV-1 Plasma Deposition of Inorganic Compounds, Chapter IV-2 Glow Discharge Polymerization, which is incorporated herein by reference. Briefly, a glow discharge plasma is generated on an electrode that may be smooth or have pointed projections. Traditionally, a gas inlet introduces high vapor pressure monomeric gases into the plasma region wherein radicals are formed so that upon subsequent collisions with the substrate, some of the radicals in the monomers chemically bond or cross link (cure) on the substrate. The high vapor pressure monomeric gases include gases of CH4, SiH4, C2H6, C2H2, or gases generated from high vapor pressure liquid, for example styrene (10 torr at 87.4xc2x0 F. (30.8xc2x0 C.)), hexane (100 torr at 60.4xc2x0 F. (15.8xc2x0 C.)), tetramethyldisiloxane (10 torr at 82.9xc2x0 F. (28.3xc2x0 C.), 1,3,-dichlorotetra-methyldisiloxane) (75 torr at 44.6xc2x0 F. (7.0xc2x0 C.)), and combinations thereof that may be evaporated with mild controlled heating. Because these high vapor pressure monomeric gases do not readily cryocondense at ambient or elevated temperatures, deposition rates are low (a few tenths of micrometer/min maximum) relying on radicals chemically bonding to the surface of interest instead of cryocondensation. Remission due to etching of the surface of interest by the plasma competes with reactive deposition. Lower vapor pressure species have not been used in PECVD because heating the higher molecular weight monomers to a temperature sufficient to vaporize them generally causes a reaction prior to vaporization, or metering of the gas becomes difficult to control, either of which is inoperative.
The basic process of flash evaporation is described in U.S. Pat. No. 4,954,371, which is incorporated herein by reference. This basic process may also be referred to as polymer multi-layer (PML) flash evaporation. Briefly, a radiation polymerizable and/or cross linkable material is supplied at a temperature below a decomposition temperature and polymerization temperature of the material. The material is atomized to droplets having a droplet size ranging from about 1 to about 50 microns. An ultrasonic atomizer is generally used. The droplets are then flash vaporized, under vacuum, by contact with a heated surface above the boiling point of the material, but below the temperature which would cause pyrolysis. The vapor is cryocondensed on a substrate, then radiation polymerized or cross linked as a very thin polymer layer.
According to the state of the art of making plasma polymerized films, PECVD and flash evaporation or glow discharge plasma deposition and flash evaporation have not been used in combination. However, plasma treatment of a substrate, using glow discharge plasma generator with inorganic compounds has been used in combination with flash evaporation under a low pressure (vacuum) atmosphere as reported in J. D. Affinito, M. E. Gross, C. A. Coronado, and P. M. Martin, xe2x80x9cVacuum Deposition Of Polymer Electrolytes On Flexible Substrates,xe2x80x9d Proceedings of the Ninth International Conference on Vacuum Web Coating, November 1995, ed. R. Bakish, Bakish Press 1995, pp. 20-36, and as shown in FIG. 1a. In that system, the plasma generator 100 is used to etch the surface 102 of a moving substrate 104 in preparation to receive the monomeric gaseous output from the flash evaporation 106 that cryocondenses on the etched surface 102 and is then passed by a first curing station (not shown), for example electron beam or ultra-violet radiation, to initiate cross linking and curing. The plasma generator 100 has a housing 108 with a gas inlet 110. The gas may be oxygen, nitrogen, water or an inert gas, for example argon, or combinations thereof. Internally, an electrode 112 that is smooth or has one or more pointed projections 114 produces a glow discharge and makes a plasma with the gas which etches the surface 102. The flash evaporator 106 has a housing 116, with a polymer precursor inlet 118 and an atomizing nozzle 120, for example an ultrasonic atomizer. Flow through the nozzle 120 is atomized into particles or droplets 122 which strike the heated surface 124 whereupon the particles or droplets 122 are flash evaporated into a gas that flows past a series of baffles 126 (optional) to an outlet 128 and cryocondenses on the surface 102. Although other gas flow distribution arrangements have been used, it has been found that the baffles 126 provide adequate gas flow distribution or uniformity while permitting ease of scaling up to large surfaces 102. A curing station (not shown) is located downstream of the flash evaporator 106. The monomer may be a (meth)acrylate (FIG. 1b).
Traditional methods for making non-linear optical polymers employ spin coating. In one type of spin coating, non-linear optical molecules are attached to the polymer backbone. Poling is then by heating to a temperature above a glass transition temperature to align the non-linear optical groups. However, the polymer backbone is not aligned and over time the poled groups relax to their non-poled condition which is unsuitable for long term devices. In another type of spin coating, the non-linear optical groups are simply mixed with the polymer precursor without attachment. Again, the poled groups relax over time. Another disadvantage is in making a non-linear optical polymer part of a multi-layer construction wherein the non-linear optical polymer must be physically moved or transferred to an area where the additional layer is applied, for example vacuum deposition.
Therefore, there is a need for an improved method for making non-linear optical polymers.
The present invention provides a method of making a non-linear optical polymer layer. The method includes providing a liquid polymer precursor mixture containing a plurality of non-linear optical molecules, flash evaporating the liquid polymer precursor mixture forming an evaporate, and continuously cryocondensing the evaporate on a cool substrate forming a cryocondensed polymer precursor layer and cross linking the cryocondensed polymer precursor layer thereby forming the non-linear optical polymer layer.
The flash evaporating may be performed by supplying a continuous liquid flow of the molecularly doped polymer precursor mixture into a vacuum environment at a temperature below both the decomposition temperature and the polymerization temperature of the molecularly doped polymer precursor mixture, continuously atomizing the molecularly doped polymer precursor mixture into a continuous flow of droplets, and continuously vaporizing the droplets by continuously contacting the droplets on a heated surface having a temperature at or above a boiling point of the liquid polymer precursor and of the molecular dopant, but below a pyrolysis temperature, forming the composite vapor. The droplets typically range in size from about 1 micrometer to about 50 micrometers, but they could be smaller or larger.
Alternatively, the flash evaporating may be performed by supplying a continuous liquid flow of the polymer precursor mixture into a vacuum environment at a temperature below both the decomposition temperature and the polymerization temperature of the polymer precursor mixture, and continuously directly vaporizing the liquid flow of the polymer precursor mixture by continuously contacting the liquid polymer precursor mixture on a heated surface having a temperature at or above the boiling point of the liquid polymer precursor, but below the pyrolysis temperature, forming the evaporate. This may be done using the vaporizer disclosed in U.S. Pat. Nos. 5,402,314, 5,536,323, and 5,711,816, which are incorporated herein by reference.
After condensation, the cross linking may be by any standard curing technique, including, but not limited to, radiation curing, including ultraviolet, electron beam, or x-ray, glow discharge ionization, and spontaneous thermal induced curing. In radiation curing (FIG. 1), the liquid polymer precursor mixture may include a photoinitiator. In glow discharge ionization curing, a combined flash evaporator, glow discharge plasma generator is used without either an electron beam gun or ultraviolet light.
Base polymer precursors may be monomers, oligomers, and resins, and combinations thereof. Examples of monomers include, but are not limited to, (meth)acrylate molecules, for example, hexanedioldiacrylate, and tetraethyleneglycoldiacrylate, styrene polymer precursors, and methyl styrene polymer precursors, and combinations thereof. Oligomers, include, but are not limited to, polyethylene glycol diacrylate 200, polyethylene glycol diacrylate 400, and polyethylene glycol diacrylate 600, tripropyleneglycol diacrylate, tetraethylene glycol diacrylate, tripropylene glycol monoacrylate, and caprolactone acrylate, and combinations thereof. Resins include, but are not limited to, polystyrene resins, epoxy polyamine resins, phenolic resins, and (meth)acrylic resins, and combinations thereof. Base polymers also include allyls, alkynes, and phenyl acetylene.
Non-linear optical molecules include, but are not limited to, dimethylamino nitrostilbene, methyl nitroanaline, urea and combinations thereof.
Accordingly, the present invention provides an improved method of making a non-linear optical polymer.